Dear all,
I am looking at the FM demodulator example
provided in the gnuradio examples (in the mc4020 directory)
and I have a question on the
the design parameters of the filter used to separate one
FM station before demodulating:
the sampling frequency is 20 Msps
the desired station is centered at 5.75 MHz
the designed filter (lowpass version)
has a cutoff frequency at 250 KHz
and a transition band of 800KHz
The signal is filtered and IQ demodulated and then
decimated by a factor of 125.
It seems that this filter is too wide:
an adjacent FM station (at 5.75 MHz + 200 KHz)
is not attenuated at all and its spectrum (after decimation)
is folded on the desired station spectrum.
This is clearly seen in the simple example I attach.
What am I missing here?
Thanks
Achilleas
===============================================
#!/usr/bin/env python
import sys
from gnuradio import gr
from gnuradio.wxgui import stdgui
from gnuradio.wxgui import fftsink
import wx
class filter_plus_fft (stdgui.gui_flow_graph):
def __init__(self, frame, panel, vbox, argv):
stdgui.gui_flow_graph.__init__ (self, frame, panel, vbox, argv)
IF_freq = 5.75e6
input_rate = 20.00e6
cfir_decimation = 125
cut_off = 250e3
trans_band_width = 800e3
src = gr.sig_source_f (input_rate,gr.GR_SIN_WAVE,IF_freq, 1e0)